1. Field of the Invention apparatus for forming an electrostatic image, and specifically relates to an image recording apparatus having a high voltage power source for a contact charger.
2. Description of the Prior Art
Hitherto, an electrostatic image recording apparatus has employed a corona charge method so as to supply a charge to a photosensitive drum by means of the corona discharge method. Since a conventional electrostatic image recording apparatus utilizing the corona discharge method encounters problems of an excessively high corona discharge voltage level of 5 to 10 KV and generation of ozonic gas, there has recently been developed an image recording apparatus which employs a contact charge method in which a electrostatic process is performed by using the high potential power source circuit which uses a relatively low voltage level of 1 KV to 3 KV without generation of ozonic gas.
An electrostatic image recording apparatus of the above-described type is arranged in such a manner that the charging, developing and transferring potentials are established with respect to the potential of the core of the photosensitive drum as the reference potential. Usually, the ground (hereinafter called "GND") level of the high potential power source circuit is made to be the same as the potential level of the core of the photosensitive drum. In addition the charging, developing and transferring potentials are all generated in the high potential power source circuit while using the above-described GND level as the reference.
The output voltage (hereinafter called "charging output voltage") bias to be applied to the charger is obtained by superimposing an AC (Alternating Current) bias (hereinafter called "charging AC voltage") and a DC (Direct Current) bias (hereinafter called "charging DC voltage") in order to cause the performance of charging the photosensitive member to be sufficiently exhibited. In particular, the AC bias is subjected to a constant current control in order to correspond to the environmental change in the charging load impedance. Similarly the output voltage (hereinafter called "developing output voltage") bias to be applied to the developing unit is obtained by superimposing an AC bias (hereinafter called "developing AC voltage") and a DC bias (hereinafter called "developing DC voltage").
In order to form an image on the photosensitive layer by rotating the photosensitive drum, its core is connected to the GND level of the high potential power source circuit at a sliding contact. Therefore, if foreign matter is mixed with the sliding contact portion, the contact resistance is raised to several gigaohms (10.sup.9 .OMEGA.). As a result, the reference potential of the photosensitive drum, the charger, the developing unit and the transferring device, which constitute the electrostatic image forming portion, becomes different from the GND level which is the reference potential of the high potential power source circuit. In an image forming apparatus which utilizes the corona discharge method, the AC impedance of the charger is several tens of megohms when viewed from the high potential power source circuit. Furthermore, the electric current is about hundreds of microamperes to several milli-amperes. Therefore, even if the contact resistance at the above-described sliding contact portion reaches several megohms, the charging performance of the photosensitive drum does not critically deteriorate. Furthermore, the developing output voltage bias does not deteriorate the charging output voltage bias.
However, in the case of the image recording apparatus which uses the contact charge method, the AC impedance is several megohms when the charger is viewed from the high potential power source. Furthermore, the AC impedance of the developing output voltage bias when the charging voltage generating circuit is viewed from the drum core is several megohms. Therefore, if the resistance of the sliding contact portion is hundreds of kilo-ohms to several megohms, the developing output voltage bias is undesirably introduced into the charging voltage generating circuit. As a result, the charging voltage output, and particularly, the constant current control circuit is adversely affected. Therefore, there arises a problem of generating an unsatisfactorily charged portion on the photosensitive drum causing an inferior image.
Furthermore, the above-described conventional structure does not provide means against the change in the equivalent impedance of the photosensitive member when viewed from the charger due to the frequency of the charging AC voltage. Therefore, there arises a problem of an unsatisfactory charge of the photosensitive drum if the frequency is raised due to the change in the environment or the characteristics or the like of the elements. There arises another problem of a dielectric breakdown of the photosensitive member if the frequency is lowered. Accordingly, the allowable width of the frequency of the charging AC voltage must be narrowed, causing the structure of the power supply circuit to become too complicated and resulting in an increase in the cost of the elements.
As an alternative to this, it might be feasible to employ a structure arranged in such a manner that the frequency of the charging AC voltage is positively changed for the purpose of overcoming the above-described problems.